Effect of Anisotropy Constants on Squareness Ratio in Sr–La–Co High-Performance Sintered Ferrite Magnets at Elevated Temperatures

To decrease the irreversible demagnetization of Sr-La-Co sintered ferrite magnets at elevated temperatures, it is required to remarkably improve the squareness ratio (H
_k
/H
_cJ
), where H
_k
and H
_cJ
are the knee field and coercivity, respectively. It is very important to find out the main reason behind the effects on H
_k
/H
_cJ
at high temperature. Particular attention has been paid to the effect of the ratio of anisotropy constant (K
_u2
/K
_u1
) on H
_k
/H
_cJ
for Sr
_1-x
La
_x
Fe
_12-x
Co
_x
O
₁₉
(x = 0-0.4) sintered ferrite magnets above room temperature, where K
_u2
, K
_u1
, and x are the quartic and quadratic anisotropy constants, and the substituted amount, respectively. The value of H
_k
/H
_cJ
linearly decreases as x value increases at every temperature. Furthermore, it decreases with higher temperatures. The value of K
_u2
/K
_u1
remarkably increases with increasing x value at elevated temperatures. The value of H
_k
/H
_cJ
linearly decreases in the order of x value as K
_u2
/K
_u1
increases for all the magnets above room temperature. Uniaxial anisotropy term (K
_u1
+ K
_u2
) has an effect on the orientation in the direction of easy magnetization. However, biaxial anisotropy term (K
_u2
/2) has two different directions of easy magnetization. The difference in easy direction of biaxial anisotropy makes an angle of π/4 to each other. An excessive increase in K
_u2
/K
_u1
is not necessarily effective to increase H
_k
/H
_cJ
. The decrease in H
_k
/H
_cJ
is mainly caused by the increase in K
_u2
/K
_u1
at elevated temperatures.